Can you go to the other PTO meetings (for the other schools) and plead your case? We see the same sort of attrition in our school system, but 4th grade is pretty early for parents to be burnt out of volunteering. In our school system, the K-2 PTAs are ridiculously active, and the 3-5 is still active but not insanely so. The 6-8 PTA limps along, but that's because there isn't much that needs to be done because at this age, there are a lot of student-run organizations that take care of some of the things that the PTA does in the younger grades, and parent volunteers are not needed (or welcome) in the classroom.
I learned to step back in the youngest grades because there were a ton of eager volunteers and I give my time to the 3-5 PTA and next year, when I again have a child in middle school, I'll be active in that one as well.
I think that at this point in the year, the focus should be on starting off next year with a bang. Go to meetings at the younger schools - there are probably participants who have kids at the grade 4-5 school who would be moved to help if they knew what the stakes are, or who have 3rd graders now who will expect the same level of participation and partnership in 4th grade. Publicize a year-end meeting at which officers for the next year will be elected, and be sure that the officers know that they will need to meet with the principal and teachers (or some teacher reps) before the end of the school year to put together a plan for the following year. Then the officers can work together over the summer to put together a calendar of events, recruit committee chair people, and get momentum for a kick-off meeting when school starts.
My guess would be that if things are that bad, it's a problem with the school administration. A principal who prioritizes parent involvement wouldn't allow this to happen. Things don't just fizzle out for no reason, they fizzle out because the leaders got the message that their work wasn't needed, wanted or valued. Perhaps that's something that happened recently, or perhaps it was years ago, but somehow, at some point, enough people got the message that this wasn't important that it came to a halt. If that's the case, make sure that the principal and teacher's are 100% on board with having a more active PTO and find out what their goals are. How do things like enrichment programs get funded without a PTO? How about year-end activities, family nights, a math league or science team, student-of-the-month programs, a school store, the book fair, field day, etc. happen without a PTO? Does that stuff just not happen?
Our middle school went through a lull some years ago. The principal and staff didn't really seem to value or welcome a parent-teacher organization, so it was more or less defunct. Luckily, enough passionate parents felt that they weren't ready to remove themselves from their children's education that 6 years ago, the organization was revived and there is now a strong partnership between the organization and the school. The PTO helps fund the purchase of library materials, brings in speakers to talk to parents about things like internet safety and drug use, offsets the costs of field trips, pays for school-wide enrichment programs like speakers or presenters, pays for field day, etc. Even though the students run their own clubs, fundraisers and social functions, there is still a place in their school for the level of organization and fundraising that only adults can manage.
Good luck to you!